My invention relates to digital communication arrangements and, more particularly, to the conversion of acoustic waves into digitally coded signals.
In digital communication systems, intelligence is generally conveyed in the form of pulse codes. Advantageously, the use of pulse codes permits the multiplexing of different types of signals on one communication channel. Consequently, data, video, and audio signals may be transmitted over a single digital facility. The transmission of an audio signal over the digital facility requires conversion of input acoustic waves to the digital code format of the facility. At every station of the facility, complex electronic circuitry is needed to filter, sample, digitize and encode each audio signal. The cost and complexity of the audio signal conversion circuitry has led to the development of transducers adapted to directly convert acoustic waves into digitally coded signals.
Digital microphones such as those disclosed in U.S. Pat. Nos. 3,286,032 issued Nov. 15, 1966, 3,622,791 issued Nov. 23, 1971, and 3,626,096 issued Dec. 7, 1971 are readily adapted to provide PCM coded signals directly from acoustic waves. U.S. Pat. Nos. 3,622,791 and 3,286,032 also disclose arrangements for obtaining delta modulation codes directly from acoustical waves by means of special signal difference detection arrangements. There are other forms of pulse code modulation, however, which provide improved transmission characteristics for speech signals. Differential pulse code modulation, as is well known in the art, provides a significant improvement in signal-to-noise ratio over PCM and delta modulation schemes, and adaptive differential pulse code modulation schemes exhibit even better signal-to-noise ratio characteristics. The aforementioned patents, however, do not provide arrangements that take advantage of the improved differential pulse code modulation characteristics. It is an object of the invention to provide direct acoustical to digital code conversion with improved signal transmission characteristics.